In a conventional plastic injection molding process a charge of molten plastic is injected under pressure from a molding machine through a flow path into the article-defining cavity of a mold. A standard injection molding machine uses a hydraulic ram to apply the plastic injection pressure, which may be up to 20,000 psi. The plastic flow path includes the nozzle of the injection molding machine, and within the mold the flow path may include the sprue, runners and gates, depending on the complexity of the mold.
The use of gas-assistance in plastic injection molding has attracted increasing commercial interest in recent years. The function of the gas is to penetrate the molten plastic resin and exert an internal pressure to urge the plastic into intimate contact with the walls of the article-defining cavity in the mold. The use of gas-assistance yields several advantages in plastic injection molding, including reducing or eliminating sink marks in the surface of the molded article, saving weight and resin consumption, shortening cycle times, and reducing clamp tonnages on the mold. The basic process of plastic injection molding with gas-assistance is disclosed in U.S. Pat. No. 4,101,617 to Friederich, issued July 18, 1978, and assigned to the assignee of the present invention.
One of the important challenges to the molding engineer in making effective use of gas-assistance is in determining a proper gas injection pressure. If the gas injection pressure is too low, the gas will be unable to displace the pressurized molten plastic in the flow path, and entry of the gas into the article-defining cavity of the mold will be impeded or blocked by the plastic in the flow path. If the gas injection pressure is too high, it will penetrate the mass of molten plastic but may not be containable internally of the plastic, i.e., the gas may rupture or blow-out from within the plastic mass and result in a malformed article in the mold cavity. Alternatively, the gas may be contained within the plastic, but may migrate from ribs and thick sections into thin areas where it can have deleterious cosmetic (e.g. pressure marks and shadows) and structural effects (e.g. internal strains).
The inertial resistance of the plastic to movement by the gas is therefore a factor in determining the gas injection pressure. Once the injected gas overcomes this inertial resistance it penetrates into the fluent center of the molten plastic within the article-defining cavity and performs its intended effects. Yet, if the gas injection pressure is too great, the gas cannot be contained within the mass of molten plastic and will rupture or "blow-out" the plastic, or migrate within the plastic beyond the gas channels designed into the article and manifest itself as a cosmetic and structural defect.
The objective of the inventor was to address the resistance presented by the plastic to entry of the gas into the article-defining cavity. The present invention provides a molding process and related apparatus toward the end of permitting gas assistance to be used with improved control over the penetration and containment of the gas within the injected plastic.